Cocoa, Objective C - where do I start?

I'm thinking about building some small test applications using Cocoa and Objective C. My main development experience is unfortunately with C# on Windows so I'm trying to make amends for my years of sinning. I have a pile of Linux experience as well but only at shell scripting level.

Anyone know if there are any decent books or guides around covering these technologies? The Apple Developer web site is a bit of a muddle.